Penem antibiotics represented by thienamycin have attracted attention as medicines because of their broad antimicrobial spectra:
Various processes for preparing penem antibiotics have been reported, e.g., in Kametani, Heterocycles, Vol. 17, pp. 463-506 (1982) and Shibuya, Yuki Gosei Kaqaku Kvokai Shi (Journal of Organic Synthetic Chemistry), Vol. 41, pp. 62 (1983). Among the proposed processes, a process using a 4-acetoxyzetidin-2-one derivative represented by formula (VII): ##STR4## wherein R.sup.3 represents a hydrogen atom or a protective group for a hydroxyl group, as an intermediate is advantageous for obtaining various kinds of penem antibiotics in that the compound of formula (VII) exhibits reactivity with various nucleophilic agents.
Known processes for preparing the 4-acetoxyazetidin-2one derivative of formula (VII) include those disclosed in Tetrahedron Letters, Vol. 23, pp. 2293 (1982); Tetrahedron, Vol. 39, pp. 2399 (1983); ibid, Vol. 39, pp. 2505 (1983); JP-A-59-181254; JP-A-61-50964; and JP-A-63-45251(the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, these known processes must start with expensive raw materials, and it has therefore been demanded to develop an economical process.